The present exemplary embodiments pertain to optoelectronic devices and, more particularly, to waveguide photodetectors that convert optical signals into electrical signals.
In optical communication systems, optical waveguides provide a transmission channel for guiding an optical signal produced by a light source, e.g., a laser, at one end of the system to a detector, e.g., a photodetector, at the other end of the system. The photodetector material, an active region, absorbs energy from the photons of the transmitted optical signal, which, in response, excites charge carriers, e.g., electrons and holes. With the application of a reverse bias voltage, the excited charge carriers are attracted to contacts on the photodetector, thereby creating an electrical current that corresponds to the optical signal. In this manner, the photodetector converts an optical signal into an electrical signal.
Due to its potential for being grown on top of silicon, germanium is an appropriate choice for a photodetector.
A lattice constant refers to the distance between unit cells in a crystal lattice. The lattice constant of germanium is not perfectly matched with the lattice constant of silicon; the lattice constant of germanium is slightly larger than that of silicon. The mismatch between the lattice constants of germanium and silicon presents problems for using a regular epitaxial growth (“EPI”) technique for growing crystals. Currently, two main methods have been heavily studied to make single crystal germanium film on top of silicon substrates. One method is using a buffer layer and post-process after selective epitaxial growth (“SEG”). The second method is using a rapid melt growth (“RMG”) technique. Between these two methods, RMG has better process compatibility but has a limitation on the structures that can be constructed.
In the RMG technique, single crystal germanium is not grown directly on top of the silicon. Rather, poly-germanium or amorphous germanium is deposited on an insulator that has an opening (“seed window”) to an underlying silicon layer. An insulator, such as a nitride, is deposited to surround the poly-germanium or amorphous germanium. The RMG method requires a micro-crucible formed by the insulator surrounding the deposited poly-germanium or amorphous germanium which causes melting and recrystallization of the germanium when subjected to anneal.
Optical detectors in a CMOS compatible process sequence can be accomplished inexpensively with an RMG germanium photodetector. RMG photodetectors have been built in silicon compatible processes but suffer from a couple of difficulties.
One difficulty is that the silicon seed region contaminates the germanium photodetector resulting in a silicon germanium detector rather than a pure crystalline germanium photodetector. This can result in poor responsivity of the photodetector.
Another difficulty is that the melting and subsequent recrystallization of the germanium photodetector can result in the formation of defects near the end of the photodetector affecting the yield and responsivity of the photodetector.